1. Field of the Invention
This invention relates to a method of preventing cracks which might develop at the terminal end of the welding base metal during arc welding, and to an end tab used therefor.
2. Description of the Prior Art
Lately, various types of automatic welding systems have been devised and practically used for increased efficiency of welding works, and most popular among such automatic welding methods is the so-called "submerged arc welding" method which is practiced at high speed and with high heat input. This welding method is high in efficiency, but as a great quantity of heat is applied to the weld zone, which moves at high speed, much thermal stress or strain is liable to occur along the welded joint, and occurrence of such thermal stress or strain is proportionate to the heat input. Particularly, in the case of welding practiced by adapting a crater treating end tab at the termination of the weld line, when the arc point directed toward the end tab passes the terminal end of the base metal, repression on that portion is momentarily released and rapid rotational deformation is caused by the stress produced by accumulated thermal expansion, resulting in development of an end crack. The chance of development of such cracks is high in single groove automatic welding where 100% weld penetration is made from one side alone. Various methods such as mentioned below have been proposed for preventing development of such end cracks.
According to one of such proposed methods, welding is carried out not from one direction alone, but from both ends of the base metal, so as to re-weld the crater portion. This method, however, involves the necessity of changing the welding direction and performing gauging along the crater joint. According to another method, the groove near the terminal end of the base metal is first filled by hand welding, so as to eliminate the need for 100% penetration all the way to the terminal end. According to this method, however, the terminal end portion where the groove is filled must be gauged from the back side and the portion where no back bead is present must be welded. Thus, any of these known methods requires troublesome work and extra time and labor therefor.
In the conventional automatic welding methods of the type in which 100% penetration is achieved all the way to the terminal end and an end tab for treating the crater is adapted at the termination of the weld line of the base metal to relieve the crater from the base metal to the end tab, it was impossible to perfectly prevent development of the cracks at the terminal end of the base metal. This is attributable to the fact that, as aforesaid, sharp and intense rotational deformation is caused at the terminal end of the base metal by thermal stress or strain when the welding electrode passes such portion. In order to prevent such rotational deformation, it is proposed to curb the end portion of the base metal by means of a crater treating end tab mounted at said portion or by mechanically giving a restrictive force or holdback to the end tab. However, in case an end tab alone is used for providing such curb or holdback, a considerably large-sized end tab must be mounted, and if the end tab used is not sufficiently large, the end tab itself may undergo rotational deformation to cause development of an end crack when the end tab receives welding heat from the welding electrode upon arrival of the latter at a position just above the end tab. It is indeed possible to restrain the terminal end of the base metal by giving a mechanical restrictive force to the end tab itself, but, in such a case, an angular deformation of the base metal could take place during welding, and it is hard to impart a stable compressive force laterally to the weld zone. It is thus hardly possible to obtain a stable restrictive force sufficient to prevent development of end cracks.
According to still another method proposed in Japanese Patent Publication No. 15146/1974, the terminal portion of the base metal is not perfectly restrained, but there is adapted a crater treating end tab which is capable of preventing rapid thermal deformation that could take place when the welding electrode passes said terminal portion. In this method, in order to prevent the terminal portion of the base metal from being rapidly opened out by thermal strain produced when the welding electrode passes thereover, the crater treating end tab is provided separately from the curbing end tab so as to shut off thermal stress or strain caused by heat given to the crater treating end tab, thereby to avoid rapid lowering of the restrictive force of the curbing end tab to prevent end cracks. When the welding electrode passes the terminal end portion of the base metal, there takes place at such portion a sharp thermal deformation, or so-called "rotational deformation", which involves changes in both the lateral and vertical directions relative to the weld line. In the device of said such patent, there is provided a means for giving a suitable restrictive force in the lateral direction so that no rapid lowering of restrictive force will take place, but no such restrictive force is imparted in the vertical direction at the termination of the weld line. Therefore, when the welding electrode passes the terminal end of the base metal, the area near said terminal end expands out rapidly on its side attached to the crater treating end tab and, as the restrictive force in the lateral direction is also not so strong, there takes place a deformation in the direction of rotation, resulting in development of end cracks if the heat input is large and the welding rate is high. FIG. 1 shows the configuration of an end tab according to the prior art and a condition of restriction near the terminal end of the base metal. In the arrangement shown in FIG. 1, restriction or curb in the lateral direction at the terminal end of the weld line of the base metal is provided by the weld zones 1, 2 and the end tab 3 holds the base metal while spaced away from the weld line so that it scarcely receives the influence of rapid thermal deformation produced at the terminal end of the weld line and can therefore curb the end of the base metal with a relatively stable restrictive force. However, the portion A in the vertical direction is left in a perfectly free state, so that when the welding electrode passes the terminal end of the base metal, said end expands out as shown by numeral 5, owing to thermal deformation, causing movement of the crater treating section 4 to the position indicated by numeral 6. Thus, rotational deformation is caused, owing to such rapid thermal deformation and insufficient curb in the lateral direction, resulting in development of end cracks. There is also a possibility that a clearance be formed between the crater treating end tab and the terminal end of the base metal to cause falling of molten metal therefrom when the welding electrode transfers from the terminal end of the base metal to the crater treating end tab.